1. Technical Field
The present disclosure relates to a light emitting device, and more particularly to a light emitting device that can be applied to high power light emitting diodes.
2. Description of Related Art
Currently, light emitting diodes (LEDs) that are characterized by long lifetime, small volume, low heat dissipation, low power consumption, fast response speed and single color light emission are widely used in indicator lights, bill boards, traffic lights, auto lamps, display panels, communication tools, consumer electronics and so on. Accordingly, different packaging techniques are developed for LEDs packaging, which results in different package structures of LEDs.
FIGS. 1(A) to 1(C) show a flip chip packaging process. As shown in FIG. 1(A), a plurality of conductive bumps 11 is formed on an aluminum substrate 10. Then, as shown in FIG. 1(B), an LED chip 12 is attached to the aluminum substrate 10 upside down. Finally, underfill and package processes are performed and a final package structure is shown in FIG. 1(C).
FIGS. 2(A) to 2(C) show a wire bonding packaging process. As shown in FIG. 2(A), an aluminum substrate or ceramic substrate 20 is coated with silver paste 21. Then, as shown in FIG. 2(B), a flip chip LED chip device 22 is attached to the substrate and wire bonding process is performed. Subsequently, underfill and package processes are performed and a final package structure is shown in FIG. 2(C).
In the above two packaging processes, because the molding compound and the substrate have different expansion factors, deformation and stripping are easy to happen. In addition, since the LED chip is electrically connected to the outside through the aluminum or ceramic substrate, the conductive path is very long, which results in too much heat absorption by the aluminum or ceramic substrate, thereby making the mass production difficult. Furthermore, if several chip products are applied to the aluminum or ceramic substrate, the aluminum or ceramic substrate becomes so weak that it needs to be processed before the reflow process.
FIGS. 3(A) to 3(C) and FIGS. 4(A) to 4(C) show two packaging processes which respectively use lead frames 30 and 40 and heat sinks 31 and 41. As shown in FIGS. 3(A) and 4(A), the heat sinks 31 and 41 located on the lead frames 30 and 40 are respectively coated with silver paste 32 and 42. As shown in FIGS. 3(B) and 4(B), flip chip LED chip devices 33 and 43 are respectively attached to the lead frames 30 and 40. Afterwards, a wire bonding process is performed. Subsequently, the underfill and package processes are performed and final package structures are shown in FIGS. 3(C) and 4(C).
In the above two packaging processes, high series thermal resistance may lead to reliability problem. Further, since the stack technique is used in the above two packaging processes, the packaged products may become very thick. To facilitate processes that are performed from the front side of the package structure, the depth of the recess must be shallow (as shown in FIGS. 4(A) to 4(C)). However, such a shallow recess would affect light collecting efficiency.
FIGS. 5(A) to 5(C) and FIGS. 6(A) to 6(C) show two packaging processes which use ceramic substrates and injection molded lead frames. It should be noted that an aluminum substrate must be further attached to the ceramic substrate. Thus, rather high cost of the ceramic substrate and addition of the aluminum substrate may result in higher fabrication cost. In addition, as shown in FIGS. 6(A) to 6(C), injection molding technique may result in high series thermal resistance of light emitting device. Accordingly, the light emitting device can not be used in high power LED package. As described above in FIGS. 4(A) to 4(C), there also may exist the problem of bad light collecting efficiency.
Accordingly, there is a need to develop a low thickness LED package structure which has low series thermal resistance and low packaging and application cost and can improve light collecting efficiency.